1. Field of the Invention
This invention relates to drill bits and more particularly concerns drill bits for use in drilling bore holes in underground formations.
2. Description of Prior Art
U.S. Pat. No. 2,953,354 describes a drill bit for drilling bore holes into earth formations. The drill bit includes a cutting head which has a circumferential cutting face and an end cutting face both having an axis which is spaced from the axis of rotation of the drill bit during drilling. This arrangement produces an eccentric rotation of the cutting head so that the bore-hole produced is larger in diameter than the diameter of the cutting head.
This arrangement has the advantage that it can be withdrawn from a bore-hole without damaging the cutting faces and can also be used for increasing the depth of an existing bore-hole or for reaming an existing bore-hole without damaging the cutting faces.
The application of this type of drill bit is limited however because it requires a drill core to centre and guide it during drilling operations. If the drill core breaks, if there is a pilot hole or if the underground formation being drilled is not sufficiently firm the drill bit will tend to rotate about its own axis so that the bore-hole will be the same diameter as the cutting head of the drill bit.
It is an object of the present invention to provide an improved drill bit for drilling bore holes in underground formations. It is a further object of the invention to provide a drill bit which is capable of drilling a bore hole of a diameter larger than the diameter of the drill bit and which has improved centring reliability.